As is well known, as an example of a typical circuit that on/off-controls current supply from a power source to a load by use of a semiconductor switch, there exists a circuit including a first transistor whose emitter and collector are connected between the power source and the load, a driving transistor whose collector and emitter are connected between the base of the first transistor and the ground, and a switch connected with the base of the driving transistor. In addition, in general, a capacitor is connected between the ground potential point and the connection point between the load and the first transistor. In other words, this capacitor is connected in parallel with the load.
In the well-known circuit configured in such a way as described above, when the switch is turned on, the driving transistor is turned on; as a result, the first transistor is turned on and hence a current is supplied from the power source to the load. When the switch is turned off, the driving transistor is turned off; as a result, the first transistor is turned off and hence the current supply from the power source to the load is stopped.
In this situation, when the first transistor is turned on, a rush current for charging the capacitor flows from the power source by way of the first transistor because the capacitor connected in parallel with the load is a capacitive load. Accordingly, it is required that the first transistor has a current rating with which the first transistor can withstand this rush current. However, in the steady state after the capacitor has been charged, only a current that is determined by the load flows in the first transistor; therefore, use of a transistor, as the first transistor, that has a large current rating, i.e., a large size only for withstanding the rush current becomes a hindrance to downsizing and cost saving of a product.
Accordingly, Patent Document 1 discloses a rush current limiting circuit in which in order to make it possible to on/off-control the current supply to the load by use of a transistor having a relatively small current rating, there is provided a circuit that limits a switch-on rush current without providing any effect to the steady-state current supply to the load. In the circuit disclosed in Patent Document 1, a resistor for detecting a rush current is connected between the emitter of the first transistor and the power source, and there is provided a second transistor that is connected, by way of this resistor, between the emitter and the base of the first transistor. The other configurations are the same as those in the foregoing well-known circuit.
In the conventional rush current limiting circuit disclosed in Patent Document 1, when the switch is turned on, the driving transistor is turned on; as a result, the first transistor is turned on and hence a current is supplied from the power source to the load. In this situation, a rush current for charging the capacitor connected in parallel with the load flows from the power source by way of the first transistor; however, because the resistor for detecting a rush current is connected between the power source and the first transistor, a voltage difference corresponding to the rush current is generated across the resistor.
The voltage difference across the resistor functions as a base bias voltage for the second transistor; thus, when the rush current exceeds a predetermined value, the second transistor is turned on and hence the base bias voltage for the first transistor becomes smaller; therefore, because the voltage between the emitter and the collector of the first transistor becomes larger, the voltage difference across the resistor works in such a way as to suppress the rush current from increasing. In other words, appropriate setting of the value of the resistor makes it possible to suppress the rush current that flows in the first transistor.